The lowest round ever shot in the US Open was, until 2017, also the lowest round ever shot in a major championship. However, since Branden Grace went ahead and shot the first-ever 62 in a men's major at the 2017 British Open Championship, the records are separate.
The lowest round in the US Open is 63, shot first by Johnny Miller in the final round of the 1973 US Open at Oakmont Country Club near Pittsburgh. Miller was the first player to shoot 63 in any major championship, and he went on to win.
Eight years later, both Jack Nicklaus and Tom Weiskopf shot 63 in the first round of the 1980 US Open on the Lower Course at Baltusrol Golf Club in New Jersey. Nicklaus went on to win that week, taking two majors that season. Curiously, Baltusrol has given up the most rounds of 63 or better in major championship history with four.
Vijay Singh shot 63 in the second round of the 2003 US Open at Olympia Fields near Chicago. Jim Furyk went on to win that week with a then-record 72-hole total, partially because the course played soft throughout the week and gave players a chance to score.
Justin Thomas shot 63 in the third round of the 2017 US Open at Erin Hills in Wisconsin, which also gave him Miller's record for the lowest single-round score against par in the national championship. Thomas made an eagle on the 650-yard closing par 5 on the par-72 Erin Hills layout to secure 63. However, he didn't win the championship on Sunday, as Brooks Koepka won on a record 16-under total.
A year later, Tommy Fleetwood shot 7-under 63 early in the final round of the 2018 US Open at Shinnecock Hills, smashing the best round ever shot on the venerable Southampton venue. His round marked the sixth time a 63 has been shot in the US Open but just the second time it was shot in the final round.